Hybrid corn and the unsettled question of heterosis

Abstract

George Shull’s 1908 seminal article ‘The composition of a field of maize’ marked the ‘exploitation of heterosis in plant breeding, surely one of genetics’ greatest triumphs’. Hybrid corn became a ‘symbol of American agriculture’ and ‘the paradigm for all developments of $\hbox {F}_{1}$ hybrid crop varieties and more generally breeding. But there is still no consensus on the definition of heterosis while its biological basis, causal factors and genetic mechanisms remain ‘unknown’, or at best ‘poorly understood’. It is thus logical to reverse the usual approach from the exploitation of a mysterious heterosis to the triumph of hybrid corn and focus on what breeders and geneticists do rather than on the theoretical reasons for their success. This factual approach produces surprising results: (i) hybrid corn extends the isolation technique of autogamous cereals to the allogamous maize; (ii) a ‘hybrid’ is an ordinary corn plant made reproducible by the breeder and only the breeder. It is proprietary rather than ‘hybrid’; (iii) for all practical purposes, heterosis is irrelevant; (iv) Shull justified his ‘hybrid’ breeding method by the ad hoc argument of maize ‘hybrid vigour’ which in 1914, he conflated under the name of heterosis with Edward East’s concept of physiological stimulation due to heterozygosity; (v) hybrid corn can increase yield only once and by a small margin and (vi) the huge yield gains of the last 80 years came from mass selection, a process inconsistent with the theory of heterosis. In conclusion, the enduring success of ‘hybrid’ corn was achieved at the expense of farmers, common welfare and biodiversity and dovetails with the industrial agriculture requirements of crop uniformity and breeder monopoly over reproduction. This critical understanding of the paradigm of plant breeding could have important implications for breeders and geneticists.

Appendices

Appendix

Vocabulary matters

The word ‘hybrid’ (from Latin ibridus, of mixed blood and Greek hubris, exuberance, excess) implies a positive effect of crossing on vigour. The 19th century biologists made the distinction between hybridization and crossing. The mule mixing the ‘blood’ and strength of the horse with the blood and rusticity of the ass is a hybrid.

‘In a broad sense, wrote Paul Mangelsdorf in Scientific American (1951, p. 39) all corn is hybrid for this plant is a cross-pollinated species in which hybridization between varieties and between races occur constantly. (...)’

A ‘hybrid’, then, is an ordinary corn plant. Mangelsdorf evades the issue with the claim:

‘But the hybrid corn with which we shall deal here is a planned exploitation of this natural tendency on a scale far beyond that possible in nature.’

He then goes on: ‘The biological basis of hybrid corn is a genetic phenomenon known as ‘hybrid vigour.

No! The biological basis of ‘hybrid’ corn is natural variations made reproducible by crossing pure lines. The incantatory use of ‘hybrid’, ‘hybridization’, ‘natural tendency (to hybridize)’ misleads users and readers.

Moreover, ‘hybrid’ conflates the process of making ‘hybrids’ (‘hybridizing’ – crossing!) pure lines with the biological result: heterozygous quasi clone or to simplify heterozygous clone if we deal with the biological aspect, or proprietary clone if we focus on the anthropological dimension of turning life into a commodity. James Crow’s observation derived from Ronald Fisher (1949) that hybridization is the ‘equivalent of reproducing asexually (my italics) the best individual of a segregating population’, i.e. cloning, legitimates extending the term ‘clone’ (Crow 1998) to the legally ‘homogeneous and stable’ crops of industrial agriculture.

Dolly extended cloning to mammals. Cloning reflects the two century long drive for uniformity, standardization and normalization required by the mass production of an industrial society. It also reflects the secular struggle against the free reproduction of living organisms. No property rights over varieties are possible because they are heterogeneous and changing while clones, being ‘fixed’, can be protected and patented. Modern industrial clones contrast with peasant land races or varieties (which have the character of being varied, the opposite of uniformity) produced by mass selection that has created since the beginnings of agriculture the immense wealth of cultivated biodiversity that is now in jeopardy.